During melting and/or holding of aluminium, it is known to use electromagnetic stirrers placed below the furnace vessel to obtain a stirring of the molten metal in the furnace vessel and to reduce the temperature and the concentration gradients in the molten metal and to increase the productivity of the furnace plant. Especially, it is desired to reduce overtemperatures at the upper surface of the molten metal. Overtemperature in this patent application is meant the temperature difference which prevails between the maximum temperature to which any part of the molten metal is heated during the melting or holding and the melting temperature of the molten metal. A great overtemperature often leads to metal losses by oxidation and formation of dross and slag. At the same time, the energy utilization of the process is negatively influenced. Metal losses and a low energy efficiency are a problem in so-called reverberatory furnaces where oil and gas burners heat the metal by convection and radiation.
By stirring the molten metal, the temperature and concentration gradients in the molten metal are equalized so that overtemperatures may be reduced and the energy efficiency of the process be improved. This can be illustrated in that, during electromagnetic stirring, the effective coefficient of heat conduction in the molten metal is increased more than 10 times compared with the coefficient of heat conduction in a non-stirred melt. By effective coefficient of heat conduction in this patent application is meant the coefficient of heat conduction which describes the heat transport in the melt bath taking into consideration both the conductive heat flux in the molten metal and/or the solid metal and the extra contribution in the form of the convective heat flux which is obtained in the molten metal through the stirring.
It is known to arrange, in furnace plants for melting and holding of aluminium, electromagnetic stirrers below the furnace for achieving a bottom stirring, for example from U.S. Pat. No. 4,294,435. Typically, this gives an increase of the effective coefficient of heat conduction by a factor of 25-35. However, in certain cases, economic and constructive complications arise when it is desired to arrange bottom stirrers below furnaces or near the furnace bottom according to the prior art. This is particularly noticeable in those cases where it is intended to install stirrers in existing furnace plants to increase the energy efficiency and productivity of the furnace and to reduce the temperature and concentration gradients in the molten metal. In addition, such a supplementary installation of bottom stirrers in an existing furnace plant is, in many cases, rendered difficult by the fact that the furnace is standing on a floor and that its bottom is not, without extensive rebuilding of the furnace hall, available for such an installation. It is known to arrange electromagnetic means in or near the walls which separate different melt baths in a melting furnace in order to achieve a stirring of the molten metal by pumping molten metal between the different baths. In similar manner, a stirring can be obtained by allowing electromagnetic means to act on a channel which is arranged in or near the walls of the furnace vessel and which communicates, at both ends, with the molten metal present in the furnace vessel. Further, U.S. Pat. No. 4,294,435 discloses that it would be desirable, in a furnace plant for melting and holding aluminium, to arrange electromagnetic means near the electromagnetic means arranged in the furnace walls, so-called side stirrers, which act through the wall and apply a magnetic stirrer field to the molten metal present in the furnace vessel in order to achieve a side stirring. However, it is not stated how side stirrers are to be designed or arranged to achieve an efficient stirring of the molten metal present in the furnace vessel when the furnace vessel has a large bath surface in relation to its bath depth.
Based on constructive and economic aspects, it is desirable, as stated above, to arrange electromagnetic stirrers to act through the side walls of the furnace, side stirrers, to achieve a side stirring. However, stirring by means of side stirrers which are placed in or near the wall of the furnace vessel has been considered to provide insufficient stirring in a furnace vessel, especially in a furnace vessel with a large bath surface in relation to its bath depth.
One object of the invention is to suggest a furnace plant which comprises at least one two- or multiphase electromagnetic stirrer, designed and arranged according to the invention to achieve an efficient side stirring in a furnace vessel with a large bath surface in relation to its bath depth, whereby the effective coefficient of heat conduction of the molten metal is increased by a factor of 10 or more, thus reducing the temperature and concentration gradients and increasing the productivity and energy efficiency of the furnace plant.